Field of the Invention
The present invention relates to an image forming apparatus, e.g., a copying machine, a printer, or a facsimile machine, which uses an electrophotographic system or an electrostatic recording system.
Description of the Related Art
Hitherto, an electrophotographic image forming apparatus uniformly charges the surface of an image bearing member (hereinafter referred to as “photosensitive drum”) to a desired potential with a charging member and exposes the charged surface with light in accordance with image information, to thereby form an electrostatic latent image.
Next, a developing device develops the electrostatic latent image with toner as a toner image. A transfer device transfers the toner image onto a recording material such as a sheet, and then the toner image is fixed onto the recording material with heat and pressure, to thereby obtain an output image. The toner remaining on the surface of the photosensitive drum after the transfer is collected by a cleaning member.
Charging members include a charging member of a contact charging type and a charging member of a corona charging type. In the contact charging type, the charging member having a voltage applied thereto is brought into contact with the surface of the photosensitive drum under predetermined pressing force, to thereby charge the surface of the photosensitive drum. In the contact charging type, the generation of ozone is significantly reduced as compared to that of the corona charging type.
When the charging member is brought into contact with the surface of the photosensitive drum, particles on the surface of the photosensitive drum, which have not been collected by the cleaning member, electrostatically adhere to the charging member.
When an adhering substance is accumulated on the charging member, the surface of the photosensitive drum suffers from a charging defect, with the result that an image defect, e.g., a streaked image, is generated in the toner image formed on the recording material.
In Japanese Patent Application Laid-Open No. H04-371972, a voltage, which has the same polarity as a normal voltage and is lower than the normal voltage, is applied to the charging member during non-image formation. With this, the adhering substance having an opposite polarity, which adheres to the charging member, is removed.
In Japanese Patent Application Laid-Open No. 2002-311692, a negative voltage and a positive voltage are respectively applied to the charging member during non-image formation at least once for at least a period of time required for one rotation of the charging member. With this, the adhering substance on the charging member is transferred onto the photosensitive drum to be removed.
Due to the recent increase in speed and longer operating life of a printer or a copying machine, and various use environments, stress to a toner tends to increase. When the stress to the toner increases, a reversed toner, which is charged to a polarity opposite to a normal polarity, and a negative external additive having a small particle diameter, which has been separated from the toner surface, increase to cause a problem in that bipolar particles adhere to the charging member.
In Japanese Patent Application Laid-Open No. H04-371972 and Japanese Patent Application Laid-Open No. 2002-311692, there are described the following problems. In Japanese Patent Application Laid-Open No. H04-371972, the adhering substance having an opposite polarity is removed, and hence the effect of removing the adhering substance having a normal polarity is insufficient.
In Japanese Patent Application Laid-Open No. 2002-311692, a bipolar voltage is applied. With this, a bipolar adhering substance is transferred onto the photosensitive drum. However, a power source configured to apply a bipolar charging voltage is required. With this, there is a problem of an increase in cost of the image forming apparatus.
Further, in Japanese Patent Application Laid-Open No. 2002-311692, there is disclosed a configuration in which a voltage having the same polarity as the charging voltage and ground connection are switched. For example, in a configuration in which the photosensitive drum is negatively charged, the application negative voltage state is switched to a ground connection. In this case, a potential difference between the surface potential of the photosensitive drum and the charging potential immediately after the switching is sufficiently ensured. Therefore, the positive adhering substance can be transferred. However, when a period of time elapses after the switching, the potential is fluctuated into a positive direction due to the dark decay of the surface potential of the photosensitive drum. With this, a sufficient potential difference cannot be ensured between the surface potential of the photosensitive drum and the charging potential, with the result that the positive adhering substance cannot be removed sufficiently.
In the case where a switching operation of the charging voltage is performed repeatedly, even when particles adhered to the charging member are transferred onto the photosensitive drum, the particles pass through the cleaning member to re-adhere to the charging member during a cleaning operation. The amount of particles adhering to the charging member can be reduced by extending the switching operation time of the charging voltage. However, when the operation time is extended, the photosensitive drum is degraded due to wearing and waiting time increases.